Local anesthetics (LAs) prevent or relieve pain by interrupting nerve conduction. They bind to specific receptor sites on the sodium (Na+) channels in nerves and block the movement of ions through these pores. Both the chemical and pharmacologic properties of individual LA drugs determine their clinical properties. This chapter discusses the basics of the mechanism of action of LAs, their clinical use, and systemic toxicity prevention and treatment.
Nerve conduction involves the propagation of an electrical signal generated by the rapid movement of small amounts of several ions (Na+ and potassium K+) across a nerve cell membrane. The ionic gradient for Na+ (high extracellularly and low intracellularly) and K+ (high intracellularly and low extracellularly) is maintained by a Na+-K+ pump mechanism within the nerve. In the resting state, the nerve membrane is more permeable to K+ ions than to Na+ ions, resulting in the continuous leakage of K+ ions out of the interior of the nerve cell. This leakage of cations, in turn, creates a negatively charged interior relative to the exterior, resulting in an electric potential of 60–70 mV across the nerve membrane.
Receptors at the distal ends of sensory nerves serve as sensors and transducers of various mechanical, chemical, or thermal stimuli. Such stimuli are converted into minuscule electric currents. For example, chemical mediators released with a surgical incision react with these receptors and generate small electric currents. As a result, the electric potential across a nerve membrane near the receptor is altered, making it less negative. If the threshold potential is achieved, an action potential results, with a sudden increase in the permeability of the nerve membrane to Na+ ions and a resultant rapid influx of positively charged Na+ ions. This causes a transient reversal of charge, or depolarization. Depolarization generates a current that sequentially depolarizes the adjacent segment of the nerve, thus "activating" the nerve and sending a wave of sequential polarization down the nerve membrane.
Repolarization takes place when sodium permeability decreases and K+ permeability increases, resulting in an efflux of K+ from within the cell and restoration of the electrical balance. Subsequently, both ions are restored to their initial intracellular and extracellular concentrations by the Na+-K+-adenosine triphosphate pump mechanism. Because the rapid influx of Na+ ions occurs in response to a change in the transmembrane potential, Na+ channels in the nerve are characterized as "voltage gated." These channels are protein structures with three subunits that penetrate the full depth of the membrane bilayer and are in communication with both the extracellular surface of the nerve membrane and the axoplasm (interior) of the nerve. LAs prevent the generation and conduction of nerve impulses by binding to the α subunit of the Na+ channel and preventing the influx of Na+ into the cell, halting the transmission of the ...